Reversible processes in thermodynamics

Question

These are a few basic questions of mine in thermodynamics whose answer I can't find anywhere.

$1$. How does a quasi static process serve the purpose of thermodynamic equilibrium?A quasi static process is one which occurs slowly enough so that the gas is in thermodynamic equilibrium at every instant which means i have information about the thermodynamic parameters of the gas at all times. Thermodynamic equilibrium means all the particles of the gas are the at the same thermodynamic parameters at that instant $t$. But I can't get my head around this. For example,let us consider a piston of ideal gas. At time $t=0$,if I change any parameter and the gas takes some time $\Delta t$ to adjust, I now know the parameter of the time ($t=0+\Delta t$) but what about the parameters of the gas between the time $0$ to $\Delta t$? Since we are having information about the gas at only discrete time intervals,how are we able to draw $P-V$ graphs for continuous time intervals? What is the correct way to interpret this?

$2$. If a process is infinitely slow(quasi static),why does it have to be reversible? Meaning why can we return to an exact previous state only by this infinitely slow process?Why will a rapid process not be reversible?(will be helpful if this isn't answered using concept of entropy since it should have an elementary proof).

$3$. A reversible process has to be infinitely slow but adiabatic processes are rapid enough so that heat exchange doesn't occur. In that case,how can an adiatic process be reversible?

These questions have been on my mind since I started thermodynamics but never got the opportunity to have them cleared out.

Answer 1

If you do the process so slowly that it takes 300 years to expand the gas by the piston moving, doesn't it strike you intuitively that, and every state of the process, the gas is vanishingly displaced from thermodynamic equilibrium. If 300 years is not enough for you, then consider 3000 years (or whatever it takes).

The irreversibility comes in when the gas is deforming very rapidly so that viscous stresses and forces, and irreversible heat conduction within the gas are also present. Viscous stresses dissipate mechanical energy to internal energy, and are are equivalent to fluid friction, which cannot be reversed. Heat conduction, another transport process, tends to dissipate temperature gradients, and this also can't be reversed.

Adiabatic processes are not necessarily rapid. An Adiabatic Process prevents heat flow into the system, which most often can be done by using insulation. Only slow adiabatic processes can be considered reversible.